1. Field of the Invention
The present invention relates generally to cardiac defibrillators, and in particular to modification of the current waveform applied to the myocardium by truncating the peak initial current during defibrillation countershock therapy when excessively low myocardial tissue resistance will lead to myocardial tissue destruction.
2. Background of the Invention
The last several decades have seen considerable advancement in the treatment of cardiac dysrhythmias. Particularly dramatic results have been achieved in the treatment for ventricular fibrillation. Early defibrillation systems were externally applied and, as the efficacy of the system was proven, efforts focused on providing increasingly smaller defibrillation systems that would be suitable for implantation. Examples of such automatic implantable defibrillator systems include U.S. Pat. No. 4,800,883 issued to Winstrom, and U.S. Pat. No. 5,007,422 isued to Pless et al. As these implantable automatic defibrillator systems continue to be improved, one challenge is to provide adequate energy output from the battery and capacitors of the system, while decreasing the overall size of the implanted defibrillator system. Another challenge is to provide the energy output in a reasonably safe fashion.
Experience has shown that the needed energy output of existing implantable defibrillator systems is in the range of 25-40 Joules (J) over approximately a six milliseconds (msec) period to insure consistently successful defibrillation. Contrast this to the 200-400 J of output energy necessary for the skin to skin externally applied defibrillation countershocks.
Existing implantable defibrillator systems generally accumulate the output energy in a storage capacitor system having an effective capacitance of approximately 150 microfarads (.mu.F) charged to 750 volts (V) and delivering the output energy as a defibrillation countershock pulse over approximately a six msec period through approximately 50 .OMEGA. of myocardial muscle resistance between two or more implantable discharge electrodes. For a general background on implantable cardioverter defibrillators (ICD) and cardioversion defibrillation therapies, reference is made to: Implantable Cardioverter Defibrillator Therapy: The Engineering-Clinical Interface, (M. W. Kroll, Ph.D., M. H. Lehmann, M.D. editors 1993).
Early defibrillator systems delivered a defibrillation countershock pulse as a mono-phasic waveform. Further development and research revealed that for treatment of ventricular fibrillation, a bi-phasic waveform is a more efficacious shape for a defibrillation countershock pulse. For further background on the efficacy of waveform shape, reference is made to: "Improved defibrillator waveform safety factor with bi-phasic waveforms" by Jones et al. in Am. J. Physiol.245 (Heart Circ. Physiol.14): H60-H65, 1983; and "Conduction Disturbances Caused by High Current Density Electric Fields" by Yabe et al. in Circulation Research 66: 1190-1203, 1990.
On the average, myocardial tissue resistance between any two implanted discharge electrodes has been found to be 50 ohms (.OMEGA.) and this has become the accepted average resistance of the myocardial tissue between the discharge electrodes for implantable systems. At the time of implantation, the physician will test this resistance to assess adequacy of electrode placement. If the myocardial tissue resistance between the discharge electrodes is too high then the system will not deliver enough energy to insure consistent success in defibrillating. If the myocardial tissue resistance is too low, initial peak currents will be excessive in the local area of the discharge electrodes causing temporary to permanent tissue injury to an already ailing heart.
Physician experience with implantable defibrillator systems has demonstrated that ideal patient conditions are not always encountered. Placement of electrodes in positions that will include the vast majority of the myocardium within the electrical discharge field and will not encounter adverse physical conditions is difficult to achieve in some patients. One of the primary adverse physical conditions encountered during implantation of a defibrillator system is too low of a load resistance between electrodes. Low inter-electrode resistance can cause unacceptably high peak current during delivery of a defibrillation countershock pulse that leads to tissue destruction in the heart in a zone beginning from the center of the electrical field and extending outward. An additional problem caused by high peak currents in the defibrillation countershock pulse is tissue stunning extending radially outward from the border of the destruction zone for some additional distance. For additional background on this type of high current tissue destruction, reference is made to: "Alterations Induced by a Single Defibrillation Shock Applied through a Chronically Implanted Catheter Electrode" by Barker-Voelz et al. in J. Electrocardiology 16(2): 167-180, 1983.
After implantation the myocardial tissue resistance can and does change. In addition to high peak current problems encountered during implantation, high peak current problems can also develop after the device has been implanted. Unfortunately, the system is now enclosed within the patient and if the resistance drops dangerously low, there is currently no way to detect this change. For additional background on fluctuating interelectrode myocardial tissue resistance, reference is made to the following abstract: "Serial Patch-Patch Impedance Values In An Epicardial Defibrillation System," by David Schwartzman, M.D., et. al. in Pacing And Clinical Electrophysiology, the NASPE Abstracts, 16(4): Part II; page 916, abstract number 263, April 1993.
Existing approaches to minimize the problems associated with high peak currents involve a decrease in the high peak current by decreasing the charging voltage used to charge the capacitors of the defibrillator system. If low inter-electrode resistance is encountered by a physician during implantation of a defibrillator system, the recommended course of action is to programmably decrease the initial charging voltage which the system uses to deliver a defibrillation countershock pulse through electrodes having a low inter-electrode resistance.
The disadvantage to this technique is that the maximum total output energy of the defibrillator system is compromised by decreasing the initial charging voltage. Unfortunately, the total output energy of a defibrillation countershock pulse is directly related to the efficacy of the defibrillator system. Therefore, decreasing the charging voltage in an attempt to avoid tissue destruction increases the likelihood of encountering treatment failure. For further background on the relationship between output energy and efficacy of the defibrillation countershock pulse, reference is made to: "Relationship of left ventricular mass to defibrillation threshold for the implantable defibrillator: A combined clinical and animal study" by Chapman et al. in Am. Heart J. 114: 274, 1987.
A general review of the art in the biomedical use of current limiting has revealed the following references. U.S. Pat. No. 4,811,156 issued to Kroll discloses a current limiting apparatus for use in medical monitoring devices that is designed to protect the patent from transient or spurious currents that could possibly shock the patient. The invention of the Kroll patent limits current in an implanted device to about 5 microamps as a safety maneuver to prevent shocks. Thus, by its very nature to prevent shocks, the invention of the Kroll patent is plainly not suited for use with an implantable cardioverter defibrillator which is specifically designed to deliver shocks.
In U.S. Pat. No. 5,083,562 issued to de Coriolis et al., discloses a current limiting mechanism designed to protect a semiconducting switch which uses a small capacitor from inadvertent switch activation by spurious electro-surgery or electro-cautery currents. However, the current limiting circuitry required to exclude spurious electro-surgery or electrocautery currents in the de Coriolis et al. patent simply can not be applied to the output current across the defibrillator electrodes.
U.S. Pat. No. 4,499,907 issued to Kallok et al., discloses a transvenous catheter capable of limiting the energy that the catheter would provide to the discharge electrodes at its tip. The purpose of the catheter is to be able to couple an external defibrillating system to a catheter that has been implanted through a transvenous route. External defibrillator systems provide upwards of 360 joules of energy with the intent to deliver this amount of energy through externally applied large surface area paddle electrodes at the skin interface. Electrodes applied directly to the epicardium or within the heart chambers via transvenous routes as used with implantable systems successfully defibrillate the heart with as little as or less than 40 joules of energy. Consequently, in order to adapt an external defibrillator system for intravenous use, some mechanism must be in place to limit the energy output to less than 40 joules of energy. The transvenous catheter disclosed by Kallok et al., discloses a lossy element in series with one discharge electrode and a second lossy element in parallel to the myocardium that allows for shunting of approximately 90% of the energy. Thus, the Kallok et al., invention always limits the amount of energy delivered to the discharge electrodes to 10% of the energy applied. Furthermore, the case of a physically large lossy element would preclude the use of the current limiter of Kallok et al., for implantable cardioverter defibrillators.
Finally, U.S. Pat. No. 4,969,463 to Dahl et al., teaches a device and method to overcome current limiting effects of defibrillation countershock. Dahl et al., teach the concept that high energy defibrillation creates gas formation secondary to electrolysis at the surface of the electrode sites. Formation of this gas on the surfaces increases resistance which decreases the amount of energy the defibrillation system can deliver. In order to overcome the increased resistance and current and voltage limiting effects due to electrolysis, Dahl et al., teaches a sequential gatling type electrical discharge through a series of small electrodes.
While significant improvements have been made to existing automatic implantable defibrillator systems, such systems are presently designed to operate with discharge electrodes that assume ideal physical conditions of the patient in whom the system will be implanted. In the event that a physician encounters a less than ideal electrode placement during implantation of the defibrillator system, existing techniques necessarily compromise the efficacy of the system in order to compensate for low interelectrode resistance. Accordingly, it would be advantageous to provide a technique for use with automatic implantable defibrillator systems that compensates for low inter-electrode resistance without decreasing the efficacy of the defibrillation countershock pulses.